p27 Protein Novocastra
p27 protein, also known as kinase inhibitory protein 1 (Kip1), binds to cyclin E/cdk2 complexes, but not to cdk2 alone, and is detected in purified extracts of growth-arrested cells. p27 protein constrains cell proliferation by setting the threshold level of cyclin E necessary to activate cdk2. The p27 protein is also present in proliferating cells, but only in a sequestered form when it is unavailable to interact with cyclin E/cdk2 complexes. It is likely that cyclin D complexed with catalytically inactive cdk4 is sufficient to sequester p27 protein and titrate its function. The presence of bound p27 protein in proliferating cells suggests that its role may not be restricted to inducing cell cycle arrest but to also set the cyclin E threshold for execution of the G1 to S phase transition during each mitotic cycle.